Electronic – After power supply is turned off, voltage still exists and is slowly dropping over a minute + long period

power supplyvoltage

I just bought this power supply for a project:

https://uk.farnell.com/mean-well/gst120a20-p1m/adaptor-ac-dc-20v-6a/dp/2815845

It is supposed to power two somewhat larger stepper motors (and an Arduino), so I needed something with a beefier amp rating.

After attaching it to a panel mount DC connector, I just went in with a multimeter to test the voltage and PINs, which gives me a 20V reading as expected.

However, after I remove the power supply from the wall, I still see 20V initially, and then I see the voltage dropping quite slowly, over a fairly long period; I'd say around half a volt per second, or even slower.

  1. I'm a beginner, and don't know what to make of this?
    Is this a normal phenomenon and does it have a name?
  2. Is my power supply broken or low-quality (it didn't come cheap!) Did I measure it wrong?
  3. Might this not be an issue with devices that expect a certain minimum voltage?

Best Answer

It is a normal phenomenon.

The reason it happens is because there is a capacitor which can hold a charge. Even when the power is off, it can keep it, or release it (which it does).

Sometimes you see it at a laptop connection that has in the power cable a small (ring type) LED, that glows when attached to mains, and when disconnected it slowly decreases in brightness until off; this can take also tens of seconds.

I don't know if there is an official term for this, but the releasing is called 'capacitor discharge'.

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